This invention relates to a tablet type coordinate input apparatus, and more particularly to an apparatus of the type using an elastic wave for detecting the coordinates of the position of an input pen.
Various types of coordinate input apparatus of this kind have been proposed hitherto and include (i) an aerial propagation type, (ii) a surface elastic wave type and (iii) a plate wave type. The apparatus of these types are disclosed in IEEE TRANSACTIONS ON COMPUTERS, Jun. 1970, pp. 546-548; IEEE TRANSACTIONS ON ELECTRONIC COMPUTERS, Oct. 1964, pp. 609-611; and IEEE 1981 ULTRASONIC SYMPOSIUM, pp. 167-170, respectively.
The apparatus of the type (i) is greatly affected by the environmental conditions including the temperature, and, therefore, correction of the adverse effect is required. Also, due to the fact that the apparatus is direcly affected by an obstruction such as the hand of the user, the apparatus cannot be used in a mode equivalent to the usual handwriting operation thereby giving a feeling of physical disorder to the user. In the case of the apparatus of the type (ii) too, the position of the input pen cannot be accurately identified when the user's hand touches the tablet, thereby giving also a feeling of physical disorder to the user. Although this type is suitable for application to a selective menu input apparatus applying an input by the finger touch, difficulty is encountered for its application to a tablet where a handwritten character or pattern is its input. The apparatus of the type (iii) is free from the defects of the two types (i) and (ii) described above, and the usual handwriting operation is possible. Transparent tablets utilizing this type and having a liquid crystal display unit and an EL display unit integrally combined therewith are disclosed in, for example, Japanese Unexamined Patent Publications Nos. 56-101278 and 58-14247 respectively. However, in the case of this type, difficulty is encountered for attaining stable and effective signal transmission and reception. This is because a plate wave whose propagation velocity is slightly lower than that of a longitudinal elastic wave is inevitably affected by the longitudinal elastic wave propagating earlier than that, and its phase changes inevitably with the increase in the propagation distance. Japanese Unexamined Patent Publication No. 56-101278 discloses a tablet in the form of a plate of an acrylic resin. The acrylic resin is a material in which sound propagates at a low velocity, and the tablet of this material has the advantage of improving the resolution. On the other hand, however, the large attenuation constant results in a complicated method for accurately detecting the propagation time of the plate wave, and the size of the tablet itself is restricted correspondingly.